The present invention relates generally to the detection of levels of materials in containers and, more particularly, to a method of detecting the presence of free liquids in large sealed containers of solidified liquid radioactive waste.
A serious problem encountered in virtually every aspect of the nuclear industry is the generation of low level radioactive liquid waste, generally referred to as "radwaste". One common expedient for handling such liquid radwaste in accordance with Federal regulations is to solidify the same in sealed metal drums or containers for burial in a proper underground depository. The three most widely known matrixes for solidifying liquid wastes are cement, urea formaldehyde (UF) and bitumen. However, improper solidification with any of these matrixes can lead to incomplete binding of the liquid or result in the subsequent formation of liquid on the top surface of the solidified portion following the initial set.
Generally, the present industry method of verifying the absence of free liquid under the American National Standard Institute (ANSI) guidelines is by visual examination and extensive destructive internal sectioning and examination of representative drums of the solid. Any appreciable change in formulation of the solidification process requires new testing. However, the destructive testing of a small number of representative solidified radwaste samples, because of their highly variable nature, cannot accurately verify the presence or absence of liquids within all the sealed solid waste containers. Moreover, this destructive testing procedure is slow and exposes the solid radioactive waste which must be re-contained prior to disposal. Furthermore, the opening of sealed radwaste containers is illegal in most states except under strict government audited and monitored conditions.
In light of the above, it can be appreciated that there is a need to develop a technique for nondestructively testing sealed radwaste containers to quickly and accurately verify the absence or presence of free liquid therein. A technique for determining free liquids in radwaste containers has been developed for small waste drums, i.e., drums of approximately 55 gallon capacities, and is described in an article entitled "Detection of Free Liquid in Sealed Containers Simulating Drums of Solidified Radioactive Liquid Waste" by Greenhalgh et al presented at the 3rd International Conference on Nondestructive Evaluation In The Nuclear Industry (1980), pp. 667-681. Basically, one of the procedures disclosed therein involves the application of heat at a constant rate for a predetermined time to an exterior surface of a closed radwaste container or drum. When the heat input is terminated, the surface temperature of the drum begins to decrease. If water is present adjacent the drum surface, the surface temperature of the drum in that region will fall faster than if backed by solids. Thus, after terminating the heat input, the surface temperature of the drum is quickly mapped or scanned by an infrared scanning camera. The mapped temperatures show up in different shades of light corresponding to differences of heat conductivity of the physically different materials, such as liquids and solids, in the drum.
The other procedure described in the above article involves heating the container with a temperature sensitive thermal image transducer having a transparent backing. The transducer is equipped with metal foil heating elements coated with a thermally sensitive phospher that exhibits a thermal image through the transparent backing in accordance with the heat absorption characteristics of the materials contained in the drum, thereby distinguishing between liquids and solids. In both procedures described above, the drums can be tilted to detect and measure any liquids disposed on the top surface of the solid.
While these techniques admirably serve their purposes on the smaller 55 gallon containers, which have wall thicknesses ranging from 1.087 to 1.519 millimeters, they are not satisfactory in detecting free liquids in the larger containers, such as the 50 cubic feet (374 gallons) radwaste containers having wall thicknesses of at least 6.35 millimeters. These larger wall thicknesses preclude reliable detection of the differences in heat conductivity between the solids and liquids behind the container walls. Thus, there is no known nondestructive method for determining free liquids in large waste containers of the 50 cubic feet and larger capacities.